U.S. patent application number 14/332096 was filed with the patent office on 2016-01-21 for methods for temporarily bonding a device wafer to a carrier wafer, and related assemblies.
The applicant listed for this patent is Micron Technology, Inc.. Invention is credited to Andrew M. Bayless, Neal Bowen, Sharon N. Farrens.
Application Number | 20160020129 14/332096 |
Document ID | / |
Family ID | 55075178 |
Filed Date | 2016-01-21 |
United States Patent
Application |
20160020129 |
Kind Code |
A1 |
Farrens; Sharon N. ; et
al. |
January 21, 2016 |
METHODS FOR TEMPORARILY BONDING A DEVICE WAFER TO A CARRIER WAFER,
AND RELATED ASSEMBLIES
Abstract
A method of bonding a device wafer to a carrier wafer includes
disposing a first adhesive over a central portion of a carrier
wafer, the first adhesive having a first glass transition
temperature, disposing a second adhesive over a peripheral portion
of the carrier wafer, the second adhesive having a second glass
transition temperature greater than the first glass transition
temperature, and bonding the first adhesive to active front side of
the device wafer and the second adhesive to a peripheral portion of
the front side of the device wafer. Related assemblies may be used
in such methods.
Inventors: |
Farrens; Sharon N.; (Boise,
ID) ; Bowen; Neal; (Kuna, ID) ; Bayless;
Andrew M.; (Boise, ID) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Micron Technology, Inc. |
Boise |
ID |
US |
|
|
Family ID: |
55075178 |
Appl. No.: |
14/332096 |
Filed: |
July 15, 2014 |
Current U.S.
Class: |
428/201 ;
156/314; 438/691 |
Current CPC
Class: |
B32B 5/142 20130101;
H01L 21/6835 20130101; B32B 7/12 20130101; B32B 2405/00 20130101;
H01L 2221/68381 20130101; B32B 2255/00 20130101; B32B 38/105
20130101; B32B 37/1292 20130101; B32B 2250/02 20130101; B32B
2457/14 20130101; H01L 2221/68318 20130101; H01L 2221/68327
20130101; H01L 2221/6834 20130101 |
International
Class: |
H01L 21/683 20060101
H01L021/683; H01L 21/306 20060101 H01L021/306; B32B 37/02 20060101
B32B037/02; B32B 7/12 20060101 B32B007/12; B32B 37/10 20060101
B32B037/10; B32B 37/12 20060101 B32B037/12; B32B 37/18 20060101
B32B037/18; H01L 21/304 20060101 H01L021/304; B32B 37/06 20060101
B32B037/06 |
Claims
1. A method of bonding a device wafer to a carrier wafer,
comprising: disposing a first adhesive having a first glass
transition temperature over a central portion of a carrier wafer;
disposing a second adhesive having a second glass transition
temperature greater than the first glass transition temperature
over a peripheral portion of the carrier wafer; and bonding the
first adhesive to at least a portion of a front side of a device
wafer and the second adhesive to a peripheral portion of the front
side of the device wafer and to a portion of an active surface of
the device wafer.
2. The method of claim 1, wherein disposing the first adhesive over
the central portion of the carrier wafer comprises disposing a the
thermoplastic adhesive having a glass transition temperature of
less than or equal to about 200.degree. C. over the central portion
of the carrier wafer.
3. The method of claim 2, wherein disposing the second adhesive
over the peripheral portion of the carrier wafer comprises
disposing a thermoplastic adhesive having a glass transition
temperature of less than about twenty-five degrees Celsius
(25.degree. C.) greater than the glass transition temperature of
the first adhesive.
4. The method of claim 1, wherein disposing the second adhesive
over the peripheral portion of the carrier wafer comprises
disposing a thermoplastic adhesive having a glass transition
temperature of at least about fifteen degrees Celsius (15.degree.
C.) greater than the glass transition temperature of the first
adhesive.
5. The method of claim 1, wherein disposing the first adhesive over
the central portion of the carrier wafer comprises disposing the
first adhesive over substantially the entire surface of the carrier
wafer and at least partially removing the first adhesive from the
peripheral portion of the carrier wafer.
6. The method of claim 5, wherein at least partially removing the
first adhesive from the peripheral portion of the carrier wafer
comprises removing the first adhesive from an area extending from a
periphery of the carrier wafer up to about twenty (20) mm radially
inward.
7. The method of claim 5, wherein at least partially removing the
first adhesive from the peripheral portion of the carrier wafer
comprises removing the first adhesive from an area extending from a
periphery of the carrier wafer more than about twenty (20) mm
radially inward.
8. The method of claim 5, wherein at least partially removing the
first adhesive from the peripheral portion of the carrier wafer
comprises applying a solvent to the first adhesive disposed over
the peripheral portion of the carrier wafer.
9. The method of claim 5, wherein disposing the second adhesive
over the peripheral portion of the carrier wafer comprises
disposing the second adhesive over the area of the peripheral
portion of the carrier wafer from which the first adhesive is
removed.
10. The method of claim 9, further comprising removing an outer
peripheral portion of the second adhesive with a solvent.
11. The method of claim 1, wherein disposing the second adhesive
over the peripheral portion of the carrier wafer comprises
disposing the second adhesive over the peripheral portion of the
carrier wafer to leave a gap between the second adhesive and the
first adhesive.
12. The method of claim 1, wherein disposing the first adhesive
over the central portion of the carrier wafer and disposing the
second adhesive over the peripheral portion of the carrier wafer
comprise disposing the second adhesive over the peripheral portion
of the carrier wafer and subsequently disposing the first adhesive
over the central portion of the carrier wafer and at least a
portion of the second adhesive.
13. The method of claim 12, further comprising removing a portion
of the second adhesive and the first adhesive from an outermost
area of the peripheral portion of the carrier wafer.
14. The method of claim 1, wherein bonding the first adhesive to
the at least a portion of the front side of the device wafer and
the second adhesive to the peripheral portion of the front side of
the device wafer comprises heating the first adhesive and the
second adhesive and pressing the front side of the device wafer
onto the heated first adhesive and second adhesive.
15. The method of claim 14, wherein heating the first adhesive and
the second adhesive comprises heating the first adhesive and the
second adhesive to at least about one hundred and fifty degrees
Celsius (150.degree. C.).
16. The method of claim 14, wherein pressing the front side of the
device wafer onto the heated first adhesive and second adhesive
comprises pressing the device wafer and the carrier wafer together
with between about eight (8) kN and thirty (30) kN of force.
17. The method of claim 1, further comprising reducing a thickness
of the device wafer by removing material from a backside of the
device wafer.
18. The method of claim 1, wherein bonding the second adhesive to
the peripheral portion of the front side of the device wafer
further comprises bonding the second adhesive to a lateral surface
of the device wafer surrounding the active surface.
19. A method of bonding a device wafer to a carrier wafer,
comprising: disposing a first adhesive with a first glass
transition temperature over a carrier wafer; removing the first
adhesive from a peripheral portion of the carrier wafer; disposing
a second adhesive with a second glass transition temperature higher
than the first glass transition temperature over the peripheral
portion of the carrier wafer; and bonding the first adhesive to a
central portion of a device wafer and the second adhesive to a
peripheral portion of the device wafer.
20. The method of claim 19, wherein disposing a second adhesive
over the peripheral portion of the carrier wafer comprises
disposing the second adhesive over the peripheral portion of the
carrier wafer out of contact with the first adhesive.
21. A method of bonding a device wafer to a carrier wafer,
comprising: disposing an adhesive having a first glass transition
temperature over a peripheral area of a carrier wafer; disposing
another adhesive having a second glass transition temperature lower
than the first glass transition temperature over the carrier wafer
and the adhesive having the first glass transition temperature;
removing a portion of the adhesive having the first glass
transition temperature and a portion of the another adhesive having
the second glass transition temperature from the peripheral area of
the carrier wafer; and bonding the adhesive having the first glass
transition temperature and the another adhesive having the second
glass transition temperature to a device wafer.
22. The method of claim 21, wherein disposing the another adhesive
with the second glass transition temperature lower than the first
glass transition temperature comprises disposing an adhesive with a
glass transition temperature at least about fifteen degrees Celsius
(15.degree. C.) lower than the first glass transition
temperature.
23. A method of bonding a device wafer to a carrier wafer,
comprising: disposing a first adhesive having a first glass
transition temperature over a portion of a carrier wafer; disposing
a second adhesive having a second glass transition temperature over
a portion of the carrier wafer; and bonding the first adhesive to a
central portion of an active surface of a device wafer and bonding
the second adhesive to a peripheral portion of the active surface
of the device wafer.
24. The method of claim 23, wherein bonding the second adhesive to
the peripheral portion of the active surface of the device wafer
comprises bonding the second adhesive to the peripheral portion of
the active surface of the device wafer extending at least about one
(1) mm radially inward from a lateral surface of the device wafer
surrounding the active surface.
25. An assembly, comprising: a carrier wafer; a device wafer having
an active surface facing the carrier wafer; a first thermoplastic
adhesive material between the carrier wafer and the device wafer;
and a second thermoplastic adhesive material exhibiting a higher
glass transition temperature than a glass transition temperature
exhibited by the first thermoplastic adhesive material peripherally
surrounding the first adhesive material between the carrier wafer
and the device wafer.
Description
TECHNICAL FIELD
[0001] Embodiments disclosed herein relate to fabrication of
semiconductor devices. More specifically, embodiments disclosed
herein relate to methods for temporarily bonding a device wafer to
a carrier wafer for backside processing, and to related assemblies
comprising a device wafer and carrier wafer.
BACKGROUND
[0002] Temporary bonding of a device wafer to a carrier wafer is an
act significant to wafer-level processing techniques used in
semiconductor manufacturing. The front side, which may also be
characterized as the active surface, of a device wafer is bonded to
a carrier wafer to support the device wafer and expose the backside
of the device wafer for thinning and exposure of conductive vias
(commonly termed through-silicon vias (TSVs)) formed through a
partial thickness of the device wafer, as well as passivation of
the backside of the thinned device wafer and formation of
conductive contact pads and, optionally, a redistribution layer
(RDL) thereon. After backside processing is complete, the device
wafer may be removed from the carrier wafer, diced, and
packaged.
[0003] Conventionally, a single thermoplastic adhesive is used for
temporarily bonding the device wafer to the carrier wafer. FIGS. 1A
through 1E illustrate a conventional temporary bonding process
flow. FIG. 1A shows a device wafer 100 and a carrier wafer 102 with
a layer of thermoplastic adhesive 104 disposed (e.g., spin coated)
on a surface thereof The thickness of the layer of adhesive 104 may
be chosen based on, for example, the topography of an active
surface 105 of the device wafer 100. Conventionally, the adhesive
104 is disposed to a thickness of between about 5 microns and 150
microns. As an example of a conventional device wafer
configuration, the device wafer 100 may be substantially
disc-shaped, and may be about 300 mm in diameter and have an
initial thickness of about 750 microns. The device wafer 100
includes a plurality of in-process semiconductor devices, such as
semiconductor memory, logic or processor dice, on the active
surface 105 of the device wafer 100 facing the carrier wafer 102.
The carrier wafer 102 is of a similar shape and size to the device
wafer 100. One conventional thermoplastic adhesive material that
has been used for adhesive 104 is WaferBOND.RTM. HT-10.10,
commercially available from Brewer Science, Inc., Rolla, Mo. After
application of adhesive 104, carrier wafer 102 with the layer of
adhesive 104 is baked to remove solvents from the adhesive 104,
e.g., solvents within the as-applied adhesive that facilitate
application of the adhesive.
[0004] Referring now to FIG. 1B, the device wafer 100 and the
adhesive-coated carrier wafer 102 are bonded together by applying
heat and pressure. For example, the device wafer 100 and the
adhesive-coated carrier wafer 102 may be mechanically pressed
together under elevated temperature conditions sufficient to soften
the adhesive layer 104 and promote adhesion between the device
wafer 100 and the carrier wafer 102. The temperature conditions
during bonding may be chosen to approach or exceed a glass
transition temperature (T.sub.g) of the thermoplastic adhesive
material of the adhesive layer 104. A portion of the softened
adhesive 104 may squeeze out around a periphery of an interface 106
between the device wafer 100 and the carrier wafer 102 during
bonding. Excess adhesive around the periphery of the interface 106
is removed, e.g., by dissolving the excess adhesive with a
solvent.
[0005] A backside 108 of the device wafer 100 is thinned by a
process such as abrasive grinding and/or chemical-mechanical
polishing with or without dry etching. The thinning process may
reduce the thickness of the device wafer 100 from an initial
thickness of, for example, approximately 750 microns to a thickness
of, for example, about 50 microns or less. As noted above, the
thinning process may expose ends of conductive vias in the device
wafer 100 on the backside 108 of the thinned device wafer 100.
Passivation and formation of contact pads 110 (shown in FIG. 1C),
with or without a redistribution layer (RDL) may follow
thinning.
[0006] Referring now to FIG. 1D, after thinning and backside
processing, the device wafer 100 may be removed from the carrier
wafer 102 by heating the polymer adhesive 104 (FIG. 1A) and
applying a shear force in direction 112 (i.e., a force generally
parallel to the plane of the interface 106 (FIG. 1B) between the
device wafer 100 and the carrier wafer 102). Heat-induced softening
of the adhesive 104 enables the device wafer 100 to be slid off of
the carrier wafer 102. A solvent may be used to clean (i.e.,
remove) residual adhesive from the device wafer 100, and the device
wafer 100 may then be mounted on a film frame support system
114.
[0007] As shown in FIG. 1E, the device wafer 100, after being
mounted on the film frame support system 114, may be separated, or
"singulated," into individual dice 116. The individual dice may
then be subsequently packaged or assembled with other dice and
packaged to form a semiconductor device or integrated device.
[0008] Thermal cycles occurring during processing that take place
while the device wafer 100 is bonded to the carrier wafer 102 may
cause the adhesive 104 to soften, compromising the adhesion between
the device wafer 100 and the carrier wafer 102. Furthermore,
differing rates of thermal expansion between the wafer material and
metallic features, such as TSVs, of the device wafer 100, and
differing rates of thermal expansion between the wafer material and
the adhesives may lead to warping of the bonded stack (i.e., the
device wafer 100 and the carrier wafer 102 bonded by the adhesive
104) during thermal cycles. During processing, portions of the
device wafer 100 may become non-parallel to the carrier wafer 102.
For example, peripheral edges of the device wafer 100 may lift away
(e.g., lose adhesion) from the carrier wafer 102. Areas of the
device wafer 100 that lift away from the carrier wafer 102 may be
thinned too much or non-uniformly during the thinning process,
impacting the ability to use dice subsequently singulated from the
device wafer 100 for three-dimensional integration in the form of
stacked die assemblies. Lifting of edges of the device wafer 100
may also cause the device wafer 100 to crack or chip and may lead
to loss of dice or of an entire device wafer 100, significantly
reducing yield.
[0009] Moreover, adhesive material 104 that flows from the
interface 106 between the device wafer 100 and the carrier wafer
102 may contaminate equipment used for backside processing, leading
to costly downtime while the equipment is cleaned.
[0010] One other approach to temporary bonding of device wafers and
carrier wafers is disclosed in United States Patent App. Pub. No.
2011/0308739 to McCutcheon et al. McCutcheon et al. disclose the
use of two different materials in a temporary wafer bonding
process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIGS. 1A through 1E are perspective views showing a prior
art temporary bonding method;
[0012] FIG. 2 is a perspective view of a carrier wafer with an
adhesive material according to an embodiment of the present
disclosure;
[0013] FIGS. 3A through 3I are cross-sectional views showing a
temporary bonding method according to an embodiment of the present
disclosure;
[0014] FIGS. 4A through 4F are cross-sectional views showing
another temporary bonding method according to another embodiment of
the present disclosure; and
[0015] FIGS. 5A through 5C are cross-sectional views showing yet
another temporary bonding process method to another embodiment of
the present disclosure.
DETAILED DESCRIPTION
[0016] The illustrations included herewith are not meant to be
actual views of any particular methods or devices, but are merely
idealized representations that are employed to describe embodiments
described herein. Elements and features common between figures may
retain the same numerical designation except that, for ease of
following the description, for the most part, reference numerals
begin with the number of the drawing on which the elements are
introduced or most fully discussed.
[0017] The following description provides specific details, such as
material types, material thicknesses, and processing conditions in
order to provide a thorough description of embodiments described
herein. However, a person of ordinary skill in the art will
understand that the embodiments disclosed herein may be practiced
without employing these specific details. Indeed, the embodiments
may be practiced in conjunction with conventional fabrication
techniques employed in the semiconductor industry.
[0018] With reference now to FIG. 2, a carrier wafer 200 including
an adhesive 202 according to the present disclosure is shown. The
adhesive 202 may include a first adhesive 204 disposed over a
central region of the carrier wafer 200 and a second adhesive 208
disposed over a peripheral region of the carrier wafer 200. The
first adhesive 204 may be a thermoplastic adhesive with a glass
transition temperature (T.sub.g) of less than or equal to about two
hundred degrees Celsius (200.degree. C.). The first adhesive 204
may have a shear modulus in the range of 1e5 Pascals (Pa) at the
T.sub.g. As a non-limiting example, the first adhesive may be
WaferBOND.RTM. HT-10.10 available from Brewer Science, Inc. of
Rolla, Mo. Other suitable materials for the first adhesive may
include, for example, WaferBOND.RTM. HT-20.20, also available from
Brewer Science, Inc., and TA13018A, likewise available from Brewer
Science Inc.
[0019] The second adhesive 208 may be a thermoplastic material with
a T.sub.g higher than the T.sub.g of the first adhesive material
204. For example, the second adhesive 208 may have a T.sub.g of
about fifteen degrees Celsius (15.degree. C.), or more, higher than
the T.sub.g of the first adhesive 204. As a non-limiting example,
the T.sub.g of the second adhesive 208 may be between about fifteen
degrees Celsius (15.degree. C.) and about twenty-five degrees
Celsius (25.degree. C.) greater than the T.sub.g of the first
adhesive 204. Suitable materials for the second adhesive 208 may
include, without limitation, 9001A, available from Brewer Science,
Inc., TA7000M available from Shin Etsu, Tokyo, Japan, or other
adhesive materials having the desired characteristics.
[0020] Since the second adhesive 208 has a higher T.sub.g than the
first adhesive 204, the second adhesive 208 may be used to function
as a dam around an outer periphery of the carrier wafer 200,
preventing flow and seepage of the first adhesive 204 from an
interface between a device wafer and carrier wafer 200 under
elevated temperatures encountered during processing of a device
wafer bonded to carrier wafer 200.
[0021] The first adhesive 204 may be chosen in part based on a
desired device feature density and topography of a device wafer to
which the carrier wafer 200 is intended to be bonded. For example,
a relatively lower viscosity adhesive may be suitable for a device
wafer with a relatively higher feature density or higher
topography, as the relatively lower viscosity adhesive may more
easily wet and fill the fine features of the semiconductor device
wafer.
[0022] The first adhesive 204 and the second adhesive 208 may be
disposed to a thickness of, for example, between about fifty (50)
microns and about one hundred fifty (150) microns. As a further
non-limiting example, the first adhesive 204 and the second
adhesive 208 may be disposed to a thickness of between about ninety
(90) microns and about one hundred (100) microns. The first
adhesive 204 may be formed on the carrier wafer 200 by conventional
techniques, such as by spin coating, dry film lamination, or spray
coating. The second adhesive 208 may be formed on the outer
periphery of the carrier wafer 200 by conventional techniques, such
as spraying, dispensing from a tip dispensing device, e.g., a
syringe, or by other methods.
[0023] The carrier wafer 200 may have a substantially circular
shape. As a non-limiting example, the carrier wafer 200 may have a
diameter of about three hundred (300) mm and an initial thickness
of about six hundred and fifty (650) microns.
[0024] Now referring to FIGS. 3A through 31, a process flow
according to an embodiment of the present disclosure for
temporarily bonding a device wafer to a carrier wafer is shown and
described. FIG. 3A shows a cross-sectional view of a portion of a
carrier wafer 300. The carrier wafer 300 includes a bonding surface
302 with a central portion 304 and a peripheral portion 306
adjacent a peripheral edge 308 of the carrier wafer 300. As shown
in FIG. 3B, a first adhesive 310 may be applied to the bonding
surface 302 of the carrier wafer 300. The carrier wafer 300 is
shown coated with the first adhesive 310. The first adhesive 310
may be chosen from the materials discussed above with reference to
first adhesive 204 (FIG. 2), or other suitable materials. The first
adhesive 310 may be disposed over substantially the entire bonding
surface 302, i.e., the first adhesive 310 may be disposed over the
central portion 304 of the bonding surface 302 and the peripheral
portion 306 of the bonding surface 302. As a non-limiting example,
the first adhesive 310 may be disposed on the bonding surface 302
by spin coating. Surface tension within the first adhesive 310 may
result in formation of a bead (i.e., a meniscus) 312 adjacent the
peripheral edge 308 of the carrier wafer 300.
[0025] A portion of the first adhesive 310 may be removed from the
peripheral portion 306 of the carrier wafer 300, as shown in FIG.
3C. The portion of the first adhesive 310 removed from the
peripheral portion 306 of the carrier wafer 300 may include at
least a portion of the bead 312. For example, FIG. 3C shows the
carrier wafer 300 after the portion of the first adhesive 310 has
been removed. As a non-limiting example, the first adhesive 310 may
be removed from an area extending from the peripheral edge 308
inward toward the central portion 304 (i.e., radially inward) up to
about twenty (20) mm. In other embodiments, the first adhesive 310
may be removed from an area extending from the peripheral edge 308
inward toward the central portion 304 more than about twenty (20)
mm. That portion of the first adhesive 310 may be removed by, e.g.,
dissolving the first adhesive 310 in a solvent or other edge bead
removal process (e.g., dry etch, mechanical abrasion, or other).
One example of a suitable solvent for removing WaferBOND.RTM.
HT-10.10 adhesive may be dodecene. Other solvents may also be
suitable depending on the material used as the first adhesive 310.
Alternatively or additionally, the first adhesive 310 may be
removed mechanically.
[0026] Alternatively, the first adhesive 310 may be disposed over
the central portion 304 of the carrier wafer 300 such that the
peripheral portion 306 remains uncoated with the first adhesive
310. For example, a spin coating process may be tailored by
adjusting, e.g., adhesive volume and spin parameters, to dispose
the first adhesive 310 over the central portion 304 of the carrier
wafer 300 while leaving the peripheral portion 306 uncoated,
substantially as depicted in FIG. 3C.
[0027] Referring now to FIG. 3D, a second adhesive 314 may be
disposed on the peripheral portion 306 of the carrier wafer 300.
The second adhesive 314 may be chosen from the materials discussed
above with reference to the second adhesive 208 (FIG. 2), or other
suitable materials. As a non-limiting example, the second adhesive
314 may be disposed over the area of the peripheral portion 306 of
the carrier wafer 300 from which the first adhesive 310 has been
removed. A secondary bead 316 of the first adhesive 310 may form
adjacent an outer edge 318 of the first adhesive 310, e.g., as a
result of surface tension within the first adhesive 310. The second
adhesive 314 may be disposed on the peripheral portion 306 by
spraying, dispensing from a tip dispensing device such as a
syringe, or by other methods.
[0028] The second adhesive 314 may be placed to leave a gap 320
between the first adhesive 310 and the second adhesive 314, i.e.,
such that the first adhesive 310 and the second adhesive 314 do not
contact one another. A width w of the gap 320 may be selected based
on the size of the secondary bead 316, the thickness of the first
adhesive 310, or other factors, and will be discussed further below
in connection with FIG. 3G.
[0029] A portion of the second adhesive 314 may be removed from the
peripheral portion 306 of the carrier wafer 300. For example, as
shown in FIG. 3E, a portion of the second adhesive 314 may be
removed from the peripheral portion 306 of the carrier wafer 300.
As a non-limiting example, a portion of the second adhesive 314 may
be removed to expose an area of the bonding surface 302 extending
from the peripheral edge 308 inward toward the central portion 304
(i.e., radially inward) about three hundred micrometers (300 .mu.m)
to three millimeters (3 mm). The portion of the second adhesive 314
may be removed, for example, using a solvent, e.g., dodecene,
p-Methane, PGMEA, or another suitable solvent.
[0030] Following removal of a portion of the second adhesive 314,
the carrier wafer 300 may be heated, e.g., baked, to remove
undesirable residual solvents, e.g., solvents that may have been
used to facilitate application of the adhesives, and/or solvents
used to remove portions of the first adhesive 310 and the second
adhesive 314. Baking time and temperature may depend on the type
and amount of the residual solvents.
[0031] Referring now to FIG. 3F, a device wafer 324 may be aligned
with the carrier wafer 300. The device wafer 324 includes a front
side 325 (the front side 325 may also be characterized as an active
side) with an active surface 326 and a peripheral area 328 having a
surface from and parallel to front side 325, with a reduced
thickness relative to a thickness of the device wafer 324 at the
active surface 326. The active surface 326 may include active
semiconductor components, for example and without limitation,
memory or logic circuitry at die locations on the active surface
326. The device wafer 324 may have a first thickness T.sub.1 from a
backside surface 332 to the active surface 326, and a second
thickness T.sub.2 from the backside surface 332 to the peripheral
area 328 of the front side 325. Such a configuration of the device
wafer 324 may be referred to in the industry as a "device edge
trim." As non-limiting examples, the device wafer 324 may be
substantially circular and have a diameter of about 300 mm, and the
first thickness T.sub.1 may be about seven hundred fifty (750)
microns. As a further non-limiting example, the second thickness
T.sub.2 may be about one hundred (100) microns less than the first
thickness T.sub.1.
[0032] The device wafer 324 may be bonded to the carrier wafer 300
under heat and pressure. For example, the device wafer 324 may be
concentrically aligned with the carrier wafer 300 by conventional
techniques, and the device wafer 324 and the carrier wafer 300 may
be placed in a mechanical press and pressed together under elevated
temperature conditions, such as at a temperature about equal to the
T.sub.g of the second adhesive 314 but higher than the T.sub.g of
the first adhesive 310. As a non-limiting example, the temperature
may be raised to at least about one hundred and fifty degrees
Celsius (150.degree. C.), and the device wafer 324 and the carrier
wafer 300 may be pressed together under at least about eight (8) kN
up to about thirty (30) kN of force for about two (2) minutes.
Bonding conditions of time, temperature, and force may be optimized
for viscous interface conditions that depend upon melt viscosity of
the first adhesive 310 and the second adhesive 314 at the bond
temperature and the density and topology of the device
architecture. As the device wafer 324 and the carrier wafer 300 are
pressed together under the elevated temperature conditions, the
first adhesive 310 melts and flows to fill and protect the
topography of active surface 326 of device wafer 324 while bonding
the active surface 326 to carrier wafer 300, while the second
adhesive 314 may soften without excessive flow (i.e., seepage),
bonding the carrier wafer 300 to the peripheral area 328 of the
device wafer 324 and in some embodiments to lateral surface 336
extending from active surface 326 to peripheral area 328. Under
these conditions, the adhesive material within the secondary bead
316 (FIG. 3D) may flow and fill the gap 320 (FIG. 3D) between the
first adhesive 310 and the second adhesive 314 as the available
volume between device wafer 324 and carrier wafer 300 decreases.
The active surface 326 of the device wafer 324 after bonding is
substantially parallel to the bonding surface 302 of the carrier
wafer 300 and the first adhesive 310 is substantially evenly
distributed between the bonding surface 302 of the carrier wafer
300 and the active surface 326 of the device wafer 324, as shown in
FIG. 3G. As also shown in FIG. 3G, the softened, higher glass
transition temperature second adhesive 314 may bond to a peripheral
portion of the device wafer 324. For example, the second adhesive
314 may bond to at least a portion of the peripheral area 328 and
to a peripheral portion of the active surface 326 of the device
wafer 324 to form a dam structure containing the first adhesive 310
between carrier wafer 300 and device wafer 324. The second adhesive
may not tend to lift the peripheral area 328 of device wafer 324,
as the outer boundary of second adhesive 314 is unconstrained and
allows peripheral expansion of the second adhesive 314 as device
wafer 324 and carrier wafer 300 are pressed together.
[0033] Referring now to FIG. 3H, material may be removed from the
backside surface 332 of the device wafer 324. For example, the
backside surface 332 may be subjected to a thinning process, such
as abrasive grinding, chemical-mechanical polishing, wet etch, or
combinations thereof. A thickness of material equal to or greater
than the thickness T.sub.2 (FIG. 3F) may be removed from the
backside surface 332 of the device wafer 324. In other words, the
backside surface 332 may be thinned to the extent that the
peripheral area 328 (FIG. 3F) of the device wafer 324 is completely
removed, leaving only a portion of the device wafer 324 including
the active surface 326, as shown in FIG. 3H.
[0034] Referring now to FIG. 31, after thinning of the backside
surface 332, excess second adhesive material 314 may be removed as
described above (e.g., by application of a solvent) to leave a
relatively small portion 334 of the second adhesive 314 extending
between the bonding surface 302 of the carrier wafer 300 and the
active surface 326 of the device wafer 324, as shown in FIG.
31.
[0035] When backside processing of the device wafer 324 is
complete, the device wafer 324 may be debonded from the carrier
wafer 300 by heating to a temperature above the T.sub.g of the
first adhesive 310 where the shear modulus of the first adhesive
310 is less than or equal to about 1e5 Pa. to reduce the adhesive
bond between device wafer 324 and carrier wafer 300. A mechanical
shear force may be applied substantially parallel to the bonding
surface 302 and the active surface 326 of the device wafer 324 to
slide the carrier wafer 300 and the device wafer 324 apart. Because
the remaining small portion of second adhesive 314 only bonds to a
small area at the periphery of carrier wafer 300 and the active
surface 326 of the device wafer 324, the cumulative bonding
strength (i.e., the total bonding strength over the entire bonding
area) of the second adhesive 314 may be negligible compared to the
cumulative bonding strength of the heated, melted first adhesive
310. Thus, the force and temperature utilized to debond the device
wafer 324 from the carrier wafer 300 using methods of the present
disclosure may be similar to that utilized for debonding when only
a single adhesive is used, e.g., a process flow similar to that
described above in connection with FIGS. 1A through 1E.
[0036] The higher T.sub.g of the second adhesive material 314
relative to the first adhesive material 310 may enable the second
adhesive material 314 to maintain its integrity during backside
processing acts that involve relatively high temperature (e.g., in
excess of about 200.degree. C.) thermal cycles. The portion 334 of
the second adhesive 314 may prevent the first adhesive 310 from
flowing from between the carrier wafer 300 and the device wafer 324
under elevated temperatures during backside processing. Eliminating
the flow may protect the active surface 326 of the device wafer 324
from adhesive squeeze out (i.e., seepage) and contamination during
thinning and backside processing acts.
[0037] Moreover, adhesive forces between the second adhesive 314,
the device wafer 324, and the bonding surface 302 of the carrier
wafer 300 may prevent heat-induced warping in the device wafer 324
from causing a peripheral edge of the device wafer 324 to lift away
from the bonding surface 302 of the carrier wafer 300. During the
thinning and backside processing, the second adhesive 314 may
provide sufficient adhesion between the device wafer 324 and the
carrier wafer 300 at temperatures that reduce the adhesive strength
of first adhesive 310 to reduce or prevent such warping. The
disclosed methods may provide increases in thickness uniformity of
the thinned device wafer 324, prevent crack formation in the device
wafer 324, and increase yields by a reduction in edge die loss.
[0038] The disclosure includes a method of bonding a device wafer
to a carrier wafer, the method comprising disposing a first
adhesive having a glass transition temperature over a central
portion of a carrier wafer. A second adhesive having a glass
transition temperature greater than the first glass transition
temperature is disposed over a peripheral portion of the carrier
wafer. The first adhesive is bonded to at least a portion of a
front side of a device wafer and the second adhesive is bonded to a
peripheral portion of the front side of the device wafer and to a
portion of an active surface of the device wafer.
[0039] The disclosure also includes a method of bonding a device
wafer to a carrier wafer, the method comprising disposing a first
adhesive with a first glass transition temperature over the carrier
wafer. The first adhesive is removed from a peripheral portion of
the carrier wafer, and a second adhesive with a second glass
transition temperature higher than the first glass transition
temperature is disposed over the peripheral portion of the carrier
wafer. The first adhesive is bonded to a central portion of a
device wafer and the second adhesive is bonded to a peripheral
portion of the device wafer.
[0040] Referring now to FIGS. 4A through 4F, another process flow
for bonding a device wafer to a carrier wafer according to an
embodiment of the disclosure is shown and described. In FIG. 4A, a
carrier wafer 400 including a bonding surface 402 is shown. The
carrier wafer 400 may be similar to the carrier wafer 300 described
above in connection with FIGS. 3A through 31.
[0041] Referring now to FIG. 4B, a second adhesive 314 may be
applied to a peripheral portion 406 of the bonding surface 402 of
the carrier wafer 400. The second adhesive 314 may be applied by,
for example, spraying, dispensing from a tip dispensing device such
as a syringe, or by other methods. The second adhesive 314 may be
selected as described above in connection with FIGS. 3A through 3I.
The area of the peripheral portion 406 over which the second
adhesive 314 is disposed may extend from adjacent a peripheral edge
408 inward toward a central portion 404 of the bonding surface 402,
for example, up to about twenty (20) mm inward from the peripheral
edge 408 (i.e., radially inward). In other embodiments, the area of
the peripheral portion 406 over which the second adhesive 314 is
disposed may extend from adjacent a peripheral edge 408 inward
toward a central portion 404 of the bonding surface 402 by more
than about twenty (20) mm. As a non-limiting example, the second
adhesive 314 may be disposed to a thickness of between about fifty
(50) to one hundred fifty (150) microns.
[0042] Referring now to FIG. 4C, a first adhesive 310 may be
disposed over substantially the entire bonding surface 402 of the
carrier wafer 400, including the peripheral portion 406 over which
the second adhesive 314 is disposed. The first adhesive 310 may be
a material as described in connection with FIGS. 3A through 3I and
may be applied to the carrier wafer 400 by conventional techniques,
such as by spin coating.
[0043] As shown in FIG. 4D, an excess portion of the first adhesive
310 and the second adhesive 314 may be removed from the bonding
surface 402 of the carrier wafer 400. For example, the first
adhesive 310 and the second adhesive 314 may be dissolved by a
solvent, such as dodecene, to leave an outermost area 412 of the
bonding surface 402 substantially free of the first adhesive 310
and the second adhesive 314.
[0044] As shown in FIGS. 4E and 4F, a device wafer 424 similar to
the device wafer 324 described in connection with FIGS. 3A through
3I may be bonded to the carrier wafer 400 in a manner similar to
that described with reference to FIGS. 3F and 3G. Remaining process
acts, such as thinning a backside surface 432 and removing excess
of the second adhesive 314, may be carried out in a manner similar
to that described with reference to FIGS. 3H and 31. The device
wafer 424 may be debonded from the carrier wafer 400 in a manner
similar to that previously described.
[0045] The disclosure includes a method of bonding a device wafer
to a carrier wafer, the method comprising disposing an adhesive
having a first glass transition temperature over a peripheral area
of the carrier wafer. Another adhesive having a second glass
transition temperature lower than the first glass transition
temperature is disposed over the carrier wafer and the adhesive
having a first glass transition temperature. A portion of the
adhesive having a first glass transition temperature and a portion
of the another adhesive having a second glass transition
temperature is removed from the peripheral area of the carrier
wafer. The adhesive having a first glass transition temperature and
the another adhesive having a second glass transition temperature
are bonded to a device wafer.
[0046] The area of the carrier wafer over which the second adhesive
is disposed may be chosen to provide a particular desired bond
strength at the periphery of the device wafer. In some embodiments,
the second adhesive may be disposed over an area of the carrier
wafer such that a greater portion of the second adhesive is
disposed between the active surface of the front side of the device
wafer and the carrier wafer compared to the embodiments of FIGS. 3A
through 3I and 4A through 4F. For example, the method shown in
FIGS. 5A through 5C may be similar to the methods shown in FIGS. 3A
through 3I and 4A through 4F, but the second adhesive may be
disposed over a wider area, such that a greater portion of the
second adhesive is disposed between the carrier wafer and an active
surface of the device wafer.
[0047] Referring now to FIG. 5A, in an additional embodiment, the
second adhesive 314 may be disposed over a peripheral portion 506
of a bonding surface 502 of a carrier wafer 500 such that the
second adhesive 314 bonds to a peripheral area 508 of an active
surface 526 of a device wafer 524. The second adhesive 314 may
extend a distance d inward toward a central portion 504 of the
carrier wafer 500 (i.e., radially inward) from a lateral surface
534 surrounding the active surface 526 of the device wafer 524. As
non-limiting examples, the distance d may be between about four
millimeters (4 mm) and about twenty millimeters (20 mm).
[0048] As shown in FIG. 5B, the device wafer 524 may be thinned as
described above in connection with FIG. 3H. Following thinning, an
excess portion 510 of the second adhesive 314 may be removed to
leave an outermost area 512 of the bonding surface 502
substantially free of the second adhesive, as shown in FIG. 5C.
[0049] In the embodiment of FIGS. 5A-5C, the second adhesive 314
disposed between the active surface 526 of the device wafer 524 and
the bonding surface 502 of the carrier wafer 500 over the
peripheral distance d (FIG. 5A) may provide increased bond strength
adjacent the lateral surface 534 of the device wafer 524 compared
to the configurations shown and described in FIGS. 3A through 3I
and FIGS. 4A through 4F. Such increased bond strength may be
desirable when the device wafer 524 is particularly prone to
heat-induced warping. For example, a device wafer 524 with a
relatively high density of features on the active surface 526 may
be more prone to warping than a device wafer with a relatively
lower feature density. The peripheral distance d, and thus the area
over which the second adhesive 314 acts to bond the active surface
526 to the bonding surface 502, may be chosen based on the
propensity of the device wafer 524 to warp during heat cycles. For
example, a relatively greater peripheral distance d may be chosen
for an application involving a wafer device more prone to edge
peeling due to heat-induced warping than would be chosen for an
application involving a wafer device less prone to such edge
peeling. Tailoring the peripheral width and strength of the second
adhesive 314 to the properties of a device wafer affecting its
tendency to warp enables effective device wafer warpage management
through selective bond strength. In addition, second adhesive 314
may be tailored with respect to its function as a dam structure in
terms of a slightly higher thermal stability in terms of higher
melt viscosity to preclude seepage of first adhesive 310.
[0050] Further, first adhesive 310 may be optimized to provide
interface properties between a device wafer and a carrier wafer
with respect to topography and conductive elements (e.g., solder
bumps) combined with favorable debonding parameters such as
debonding temperature and device wafer slide speed.
[0051] Superior adhesion of the device wafer to the carrier wafer
by using a two (inner and outer) zone adhesive bonding approach
device may eliminate wafer edge chipping, device wafer
delamination, crack formation, and loss of die. Overall thickness
uniformity of the thinned device wafer is increased, increasing
yield by reducing edge die loss. Other processing steps may also be
simplified since the incoming device wafer/carrier wafer stack is
flat. As a non-limiting example, abrasive processing, such as CMP
processing, and device wafer backside plating each benefit from
flat, well-adhered device wafer/carrier wafer stacks.
[0052] The disclosure includes a method of bonding a device wafer
to a carrier wafer, the method comprising disposing a first
adhesive having a first glass transition temperature over a portion
of the carrier wafer. A second adhesive having a second glass
transition temperature is disposed over a portion of the carrier
wafer. The first adhesive is bonded to a central portion of an
active surface of a device wafer, and the second adhesive is bonded
to a peripheral portion of the active surface of the device
wafer.
[0053] The disclosure also includes an assembly including a carrier
wafer and a device wafer having an active surface facing the
carrier wafer. A first thermoplastic adhesive material is between
the carrier wafer and the device wafer. A second thermoplastic
material exhibiting a higher glass transition temperature than a
glass transition temperature exhibited by the first thermoplastic
adhesive material peripherally surrounds the first adhesive
material between the carrier wafer and the device wafer.
[0054] While certain illustrative embodiments have been described
in connection with the figures, those of ordinary skill in the art
will recognize and appreciate that embodiments encompassed by the
disclosure are not limited to those embodiments explicitly shown
and described herein. Rather, many additions, deletions, and
modifications to the embodiments described herein may be made
without departing from the scope of embodiments encompassed by the
disclosure, such as those hereinafter claimed, including legal
equivalents. In addition, features from one disclosed embodiment
may be combined with features of another disclosed embodiment while
still being encompassed within the scope of the disclosure.
* * * * *